The present invention relates to semiconductor optical image sensors, and particularly, to a novel CMOS (complementary metal oxide semiconductor) image sensor photodiode structure having a deep trench with a diode all along its sidewall to increase the capacitance of the structure without increasing the area of the cell. This trench structure can be designed in such a way that the trench will be fully depleted during the reset phase of operation and can hold a larger charge due to its larger capacitance than a conventional photodiode.